Chris Good explains why the news is significant:
Because Levi Johnston represented a liability to Sarah Palin. He made repeated claims about her home life, representing her as a genuinely awful and selfish person. Regardless of whatever axe he had to grind, Palin's critics don't think she has any credibility either, and his claims were probably, mostly, believed by the people who don't support her. Even if you think he was lying, his public spat with the Palins added an unwanted bush-league, Jerry-Springer, soap-opera aspect to their private lives, and apparently it's one that Sarah Palin won't have to deal with anymore in her career as a public figure/politician.
Because she threatened to keep him from his child and longtime girlfriend. Taegan Goddard notes:
There was no mention of the book Johnston was supposedly writing that would "air the true story of the Palin household."
She's nothing if not brutal in defense of her own interests and lies.